X. G. Zheng

Direct view of stress distribution in solid by mechanoluminescence

Visualization of stress distribution has been realized by a nondestructive mechanoluminescence (ML) from SrAl2O4:Eu, which can emit three magnitudes higher visible light than that of well-known ML substance of quartz. A simulation result confirms that such a ML image successfully reflects the stress distribution. A kinetic model for ML of SrAl2O4:Eu is proposed.

Artificial skin to sense mechanical stress by visible light emission

The idea and successful practice of a stress sensor to sense mechanical stress by an artificial skin, i.e., self-diagnosis thin film, has been realized, through the fabrication of a high-luminescence thin piezoelectric film which can reproducibly emit strong visible light upon stressing. The strongest luminescent film consists of nanosized crystallites of ZnS doped with 1.5 at. % Mn, in which Mn acts as the emitting center. The intensity of the emitted luminescence responds to stress applied directly onto the film or to the underlying material reversibly and reproducibly, so it can be used as an artificial skin to sense mechanical stress.

Dynamic visualization of stress distribution by mechanoluminescence image

We report the realization of the dynamic image of stress distribution by developing a remarkably strong mechanoluminescence (ML) material of Sr0.975Al2O3.985:Eu0.01, which can emit four orders of magnitude larger intensity than that of the reported strong ML material of quartz crystal. This ML material can be mixed in the target composite or coated on the surface to sense stress by emitting visible light. This method is applicable to the dynamic visualization of stress distribution in a solid not only in the atmosphere but also in an aqueous environment.

Strong elasticoluminescence from monoclinic-structure SrAl2O4

Elastico-deformation luminescence in strontium aluminates was investigated systematically using precisely controlled pure-phase Eu-doped strontium aluminates of SrAl12O19, Sr4Al14O25, SrAl4O7, α-SrAl2O4, β-SrAl2O4, Sr3Al2O6 and their mixed phases. This study revealed that only the α-SrAl2O4 phase produces strong elastico-deformation luminescence; other strontium aluminates show no deformation luminescence. Correlation of deformation luminescence and crystal structure was found. The α-SrAl2O4 has the lowest symmetry, crystallizing in a monoclinic structure. This finding can be applied in designing strong elastico-deformation-luminescent materials.

Preparation and characteristics of highly triboluminescent ZnS film

Triboluminescence (TL) is the emission of light induced by the application of mechanical energy. The triboluminescent intensities of various inorganic bulk materials and films were investigated with a photon counting system. It was found that ZnS doped with 5 at% Mn exhibited the strongest TL intensity among the materials investigated. Increase in the crystallinity of ZnS also greatly enhanced TL intensity. Optimizing preparation conditions produced a strong triboluminescent film, which gave an intense visible light emission when friction was applied to it.

Giant negative thermal expansion in magnetic nanocrystals

Most solids expand when they are heated, but a property known as negative thermal expansion has been observed in a number of materials, including the oxide ZrW2O8 (ref. 1) and the framework material ZnxCd1-x(CN)2 (refs 2,3). This unusual behaviour can be understood in terms of low-energy phonons, while the colossal values of both positive and negative thermal expansion recently observed in another framework material, Ag3[Co(CN)6], have been explained in terms of the geometric flexibility of its metal-cyanide-metal linkages. Thermal expansion can also be stopped in some magnetic transition metal alloys below their magnetic ordering temperature, a phenomenon known as the Invar effect, and the possibility of exploiting materials with tuneable positive or negative thermal expansion in industrial applications has led to intense interest in both the Invar effect and negative thermal expansion. Here we report the results of thermal expansion experiments on three magnetic nanocrystals-CuO, MnF2 and NiO-and find evidence for negative thermal expansion in both CuO and MnF2 below their magnetic ordering temperatures, but not in NiO. Larger particles of CuO and MnF2 also show prominent magnetostriction (that is, they change shape in response to an applied magnetic field), which results in significantly reduced thermal expansion below their magnetic ordering temperatures; this behaviour is not observed in NiO. We propose that the negative thermal expansion effect in CuO (which is four times larger than that observed in ZrW2O8) and MnF2 is a general property of nanoparticles in which there is strong coupling between magnetism and the crystal lattice.

Room temperature sensing of ozone by transparent p-type semiconductor CuAlO2

A transparent p-type semiconductor, CuAlO2, shows selective and reversible response to ozone gas at room temperature. All existing commercial semiconductor ozone sensors are of n type. This study demonstrates the feasibility of developing an inexpensive p type and transparent ozone sensor. Types of transparent p–n junction ozone sensors may be fabricated using the p-type semiconductor CuAlO2 and existing n-type transparent materials such as In2O3.

Enhancement of adhesion and triboluminescence of ZnS:Mn films by annealing technique

Abstract Highly triboluminescent and strongly adherent ZnS films doped with Mn (ZnS:Mn) were fabricated on glass substrates by ion-plating and a post-plating heat treatment in a vacuum-sealed quartz glass tube. A crystalline ZnS:Mn film was prepared from a high-crystallinity ZnS:Mn pellet by ion-plating. The crystallinity and the friction-induced photon-emitting capability (triboluminescence) of the films were greatly enhanced by a post annealing in vacuum-sealed quartz glass tube. Of greater interest, we found that this annealing treatment strengthened the bond to the glass substrate together with enhancing the endurance to friction, which greatly enhanced its practicability to act as a stress/friction sensor for various applications.

Dielectric measurement to probe electron ordering and electron-spin interaction

Here we found that dielectric measurement can be a sensitive probe to detect the behavior of electrons and electron-spin interaction in a strongly electron-correlated system. The CuO single crystal samples with and without excess holes were investigated by dielectric measurements in combination with magnetic susceptibility measurement. Only for the semiconducting single crystal with holes was dynamic dielectric relaxation with prominent frequency dispersion and large dielectric constants observed, suggesting localization of holes in the lattice. Coupling of hole and spin was directly observed by dielectric anomalies at the magnetic phase transitions. A pseudoferroelectric-like characteristic in the E–P plot was also observed. This study verifies that the holes are strongly coupled both to the vibrational and magnetic degrees of freedom in CuO. It also suggests the possibility of using dielectric measurement as a tool to investigate the coupling of electron and spin in strong correlated systems.

Evidence of charge stripes, charge-spin-orbital coupling and phase transition in a simple copper oxide CuO

There is a great current interest in understanding the role of charge stripes in high- T c superconductivity. We found that static charge ordering and alignment of charge-ordered domains analogous ...